I don’t think that is quite right. The mosquitto.so file is in a similar named folder to @nick in my system (php 7.0).
In the conf.d folder there should be a symlink like
lrwxrwxrwx 1 root root 41 Feb 16 16:17 /etc/php/7.0/fpm/conf.d/20-mosquitto.ini -> /etc/php/7.0/mods-available/mosquitto.ini - I think this is created by the phpenmod command.
I think disable the add in, uninstall it and then reinstall it might be the way forward.
Are the file permissions on the mosquitto.so file as the rest of the directory?
Did you install mosquitto any other way?
Searching on Google with the error as the search term throws up a number of answers.